1. Field of the Invention
The present invention relates to the separation of tertiary butyl acetate from associated acetic acid by a distillation process wherein a substantial concentration of water is maintained in the acetic acid distillation bottoms stream thus enhancing purity of the separated tertiary butyl acetate while suppressing corrosion in the distillation column.
2. The Prior Art
Tertiary butyl acetate is chemical of commerce which is rapidly growing in sales volume and importance.
Although a number of methods are known for production of tertiary butyl acetate an especially advantageous procedure involves the catalytic reaction of isobutylene and acetic acid. See, for example, U.S. Pat. No. 5,994,578. Other processes involve reaction of acetic anhydride with tertiary butanol, and the like.
In such processes, product tertiary butyl acetate is associated with acetic acid and must be separated from the acetic acid prior to use. Frequently product mixtures also contain other impurities such as tertiary butanol, isobutylene oligomers, and the like which also must be separated.
A particular problem encountered in the distillation separation of tertiary butyl acetate and acetic acid is that under normal conditions the acetic acid is quite corrosive.
Production of t-butyl acetate is generally carried out by reacting acetic acid with isobutylene, or acetic anhydride with t-butanol, or a combination of these reactions. The reaction product in all cases contains free acetic acid along with the other reactants and reaction products and by-products such as di- and tri-isobutylene. To recover pure t-butyl acetate, the unreacted acetic acid must be separated from the reaction mass and generally recycled to the reactor.
Acetic acid being the heavy species in the reaction mass, is separated as a distillation column bottoms, while the other components are taken over as a distillate. Concentrated, boiling acetic acid generally corrodes commonly used stainless steels (304, 316) at a rate generally unacceptable for the life of the equipment. Metals from corrosion carried by the acetic acid as a recycle stream, are picked up by the ion exchange resin, if used as catalyst, reducing its life. It is thus, very desirable to reduce corrosion by acetic acid in the distillation column separating the acid.
Separation of acetic acid from the mixture containing t-butyl acetate, t-butanol, and isobutylene and its oligomers is also difficult. To reduce acetic acid to less than 100 ppm level in the distillate requires a large number of separation stages and high reflux, leading to low productivity. It is very desirable to improve this separation step.
In accordance with the invention, tertiary butyl acetate and acetic acid are separated by distillation; the distillation conditions are regulated to provide a concentration of water in the distillation column bottoms in the range of 0.1 to 5 wt %, preferably 0.3 to 0.7 wt %, to suppress corrosion, and to maintain a concentration of water in the upper part of the column to enhance the separation of acetic acid from the other components.